Magnetoresistive random access memory (MRAM) cells including magnetic tunnel junctions (MTJ) memory elements will be described as examples of devices that can be used with the method of the invention. MTJs can be designed with in-plane or perpendicular magnetization directions with respect to the film surface. The free magnetic layer in an MTJ memory element has a magnetization direction that is switchable in either of two directions. The resistivity of the whole MTJ layer stack changes when the magnetization of the free layer changes direction relative to that of the reference layer, exhibiting a low resistance state when the magnetization orientation of the two ferromagnetic layers is substantially parallel and a high resistance when they are anti-parallel. Therefore, the cells have two stable states that allow the cells to serve as non-volatile memory elements.
The MRAM cells in an array on a chip are connected by metal word and bit lines. Each memory cell is connected to a word line and a bit line. The word lines connect rows of cells, and bit lines connect columns of cells. Typically CMOS structures include a selection transistor which is electrically connected to the MTJ stack through the top or bottom metal contacts. The direction of the current flow is between top and bottom metal electrode contacts.
Reading the state of the cell is achieved by detecting whether the electrical resistance of the cell is in the high or low state. Writing the cells requires a sufficiently high DC current flowing in the direction through the MTJ stack between the top and bottom metal electrode contacts to induce a spin transfer torque that orients (switches) the free layer into the desired direction. The amount of current needed to write the cells is higher than the current that flows during the read process, so that a read operation does not change the state of the cell.
A memory element of MRAM typically consists of a bottom electrode, a MTJ (Magnetic Tunnel Junction) with a barrier layer such as MgO sandwiched between a top magnetic layer underneath a top electrode (TE) and a bottom magnetic layer on top of a bottom electrode (BE). One of the magnetic layers serves as the free layer with a switchable magnetization, and the magnetization of the reference layer remains fixed in normal operation. In a typical fabrication process the layers of the memory element/device are deposited and patterned, then the back end of line (BEOL) process is performed as a series of steps in which the top electrode is connected to a bit line. The core component of the STT-MRAM (Spin Torque Transfer Magnetic Random Access memory) is the magnetic tunnel junction (MTJ). The resistance area product (RA) and tunnel magnetoresistance (TMR) qualities of the MTJ critically affect the performance of STT-MRAM. The ability to characterize these properties (RA, TMR) during fabrication is important for quality control and reduced overall cost. In the prior art RA and TMR can be characterized both in the sheet film level (before etching) and after back end of line (BEOL) process. However, there is a need for RA and TMR characterization right after MTJ etch process before the wafer moves into BEOL process. Such a characterization ability is important for etching quality control, speeding up production evaluation and reducing development time.
A method that is preferably nondestructive is desired for obtaining timely feedback in the design/research process and for monitoring of fabrication process after the MTJ pillars have been etched on the wafer.
Worledge, et al. have described a method for measuring magnetoresistance (MR) and resistance area product (RA) of unpatterned magnetic tunnel junction film stacks. The RA is measured by making a series of four-point probe resistance measurements on the surface of an unpatterned wafer at various probe tip spacings. The probe tips are spaced apart on the order of microns for typical applications. The MR is obtained by repeating the measurement while applying different magnetic fields. (Worledge, et al.; Magnetoresistance measurement of unpatterned magnetic tunnel junction wafers by current-in-plane tunneling, Applied Physics Letters, Vol. 83, No. 1, 7 Jul. 2003, pp. 84-86; and U.S. Pat. No. 6,927,569.)
Commercially available automated metrology tools (e.g. from CAPRES), which are designed for measuring selected magnetic parameters of unpatterned MTJ film stacks, use multi-point probes with probe tip spacings in the micron range. Four- and twelve-point probes are available for these automated metrology tools. As an example, a CAPRES twelve-point probe is used with a 12-by-4 multiplexor (MUX) to select a total of 495 different pin-configurations each with different probe spacings (pitch). This approach allows the selected tests to be performed with different probe spacings without having to have the tips be movable with respect to each other. Existing automated metrology tools also provide means for applying a selected magnetic field to the test sample.